Amplifier arrangements provided in reception circuits in telecommunications technology in the context of radio frequency signal transmission normally have a multistage construction. In this case, it is customary to feed a radio frequency voltage signal into the chain of amplifier stages.
Such a multistage amplifier in a radio frequency receiver is often also concomitantly used to generate a so-called RSSI signal. The RSSI (Receive Signal Strength Indicator) signal represents the power level of the received radio frequency signal at the input of the amplifier.
For this purpose a rectified signal is normally generated at each amplified stage, said signal being present at the emitter node of the differential amplifier of the amplifier stage. The rectified signal is compared with a reference voltage by means of a further differential amplifier in each amplifier stage. The output currents of the second differential amplifier are subtracted from one another and this difference is added to the differences of the further amplifier stages. An RSSI signal is thus provided.
In the unmodulated case, i.e. given an input power of 0, the output currents are all equal and the current difference is 0. Given a maximum input power, all the second differential amplifiers are fully modulated and a maximum current difference results. What is characteristic of such an amplifier with an RSSI output is that the characteristic curve is dB-linear since the output current per gain of an amplifying stage increases by a differential current magnitude of one of the second differential amplifiers.
Furthermore, the known RSSI circuit requires an AC coupling between all the amplifier stages in order to decouple the offsets of the amplifiers, which AC coupling may be realized, for example by means of coupling capacitances between the amplifier stages or in the form of an offset control over the entire chain.
In addition to its high current requirement, what is disadvantageous about the known RSSI circuit is the fact that its temperature response and also manufacturing tolerances are problematic to compensate for and, moreover, the number of amplifier stages exponentiate the resulting error.
A further disadvantage is that the RSSI circuit, owing to the transitions between the amplifier stages, generates a ripple in the characteristic curve, which has to be linearized with additional outlay.
If the RSSI amplifier is intended to be arranged in a power control loop in a radio receiver, it is desirable for there to be a voltage-linear characteristic curve especially in the upper power range, since a particularly high accuracy is required in this range.